The present invention relates to a magnetic disk file unit; and more particularly, relates to a method and apparatus for releasing a magnetic head from a magnetic disk when starting up the magnetic disk file unit.
When a magnetic disk file unit is operated, the magnetic head floats on a thin layer of air which is caused by rotation of the disk. Precisely speaking, the magnetic head is part of a slider. In this specification, however, the term "head" refers to the entire slider including the actual magnetic head.
In these days, most magnetic disk file units employ a so-called CSS or "Contact Start Stop" system, in which the head contacts the disk surface when the disk is not rotating. This system is practical because miniaturization of the head has reduced the amount of pressure on the disk due to the head. The use of much cleaner air in the disk housing and the use of a lubricant also aid in making the system practical. The lubricant reduces dynamic friction between the head and the disk when starting and stopping the rotation of the disk and reduces damage if the head either crashs onto the disk due to mechanical shock or the head forces dust into the disk.
However, because heads continue to be further-miniaturized, and because the finish of both the head and disk surfaces are continuingly being made smoother, the head tends to adhere to the disk surface when the disk is not rotated. This adhesion is caused by a kind of attraction force which can occur between two mirror smooth surfaces. This attraction force becomes stronger with presence of the lubricant, and would cause serious damage to the head, its suspension structure or the disk if not released properly.
To ensure the release of the adhesion before starting to rotate the disk, conventional magnetic disk file units employ a specific release mechanism. This mechanism includes moving a head actuator along a radius direction of the disk before the disk starts to rotate. Since the suspension structure gimbals have enough stiffness in the radial direction, the head can be released from the adhesion without causing any damage to the gimbals.
Several ways of moving the head have been proposed. In U.S. Pat. Nos. 4,530,021 and 4,589,036, the head is supplied with an alternating current to cause "micromotion" when starting rotation of the disk. Further, in 4,589,036, actual movement of the head is sensed and the "micromotion" is controlled thereby. However, these methods require a precisely controlled alternating current generator and sophisticated sensing circuits.
Another method, which uses a much simpler circuit, is disclosed in Japanese laid open patent Tokkaisho 61-198480 (published Sept. 9, 1986). In this method, when the unit is stopped, the head actuator is positioned, close to a stopper means. The stopper means restricts the actuator movement and comprises an elastic material. Upon starting up the disk unit, the actuator is moved towards the stopper and compresses the elastic material. This causes the head to be slightly offset in the radial direction before the disk actually begins to rotate. This method requires only an actuator drive current large enough to compress the elastic material. A square waveform is used as the drive current, because the movement of the actuator is safely restricted by the stopper.
However, the inventor of present invention has discovered that use of such a simple waveform, with steep rise and fall characteristics can cause serious impact to the head and its gimbals, and in some cases, causes damage to the disk.